Inkjet printing technology is used in many commercial products such as computer printers, graphics plotters, copiers, and facsimile machines. One type of inkjet printing, known as “drop on demand,” employs one or more inkjet pens that eject drops of ink onto a print medium such as a sheet of paper. Printing fluids other than ink, such as preconditioners and fixers, can also be utilized. The pen or pens are typically mounted to a movable carriage that traverses back-and-forth across the print medium. As the pens are moved repeatedly across the print medium, they are activated under command of a controller to eject drops of printing fluid at appropriate times. With proper selection and timing of the drops, the desired pattern is obtained on the print medium.
An inkjet pen generally includes at least one fluid ejection device, commonly referred to as a printhead, which has a plurality of orifices or nozzles through which the drops of printing fluid are ejected. Adjacent to each nozzle is a firing chamber that contains the printing fluid to be ejected through the nozzle. Ejection of a fluid drop through a nozzle may be accomplished using any suitable ejection mechanism, such as thermal bubble or piezoelectric pressure wave to name a few. Printing fluid is delivered to the firing chambers from a printing fluid reservoir to refill the chamber after each ejection. An inkjet pen typically includes a standpipe that delivers printing fluid from the printing fluid reservoir to the printhead. A screen filter is disposed at the entrance of the standpipe to prevent particulate contaminants or free air in the printing fluid from reaching and clogging the printhead.
During operation, relatively cool printing fluid is drawn into the standpipe and is warmed as it flows toward the printhead. The printhead generates heat as its fluid ejectors are activated or fired to eject droplets of printing fluid through the nozzles. For a primarily water-based printing fluid, the solubility of air decreases as the printing fluid is heated. As a result, air is driven out of the printing fluid and accumulates in the standpipe. Often, the standpipe includes a chamber, referred to as the standpipe plenum, that temporarily warehouses the air. Because of the extremely small pore size of the screen filter, air does not readily pass through the filter into the printing reservoir and becomes trapped in the standpipe plenum. Over time, the standpipe plenum may be filled with sufficient air to restrict the proper flow of printing fluid. Printing under such conditions results in print defects. Moreover, the amount of air trapped in the standpipe plenum can eventually reach the point of causing complete printing fluid starvation or depriming of the printhead so as to render the inkjet pen useless.
In order to avoid depriming of the printhead, it is common to employ preemptive priming by purging the air from the standpipe plenum. Currently, such preemptive priming operations are performed based on estimates of the amount of air present in the standpipe plenum. However, predicting when the standpipe plenum will actually need to be purged of air is difficult because many factors influence the rate of air accumulation in the plenum. Typically, testing is done to characterize the time or amount of printing fluid expended between deprime events. These data are very noisy and as a consequence conservative trigger points are selected to protect the user from deprimes. This means that the majority of priming operations occur more frequently than necessary, resulting in wasted printing fluid and user delays waiting for the unnecessary priming operation to complete.